Airline industry groups and some pilot unions are blasting a government plan to set new limits on pilot workdays to reduce fatigue, according to comments released Monday.

Opposition to the proposal to set new limits on pilots' work hours was widespread in the hundreds of submissions to the Federal Aviation Administration. The agency must consider the comments before finalizing the rule, making the intense opposition a hurdle. Congress ordered the FAA to finalize new anti-fatigue standards by next Aug. 1.

Airline groups said the new rules would cost airlines billions of dollars to hire thousands of new pilots and alter schedules and would not enhance safety. Some unions and their supporters said the proposal does not go far enough to limit long workdays.

The Air Transport Association, which represents large carriers, called the proposal "onerous" and far too costly. The FAA estimated it would cost airlines $1.25 billion over 10 years, but the airline group said the total was closer to $20 billion.

"We are very concerned that the proposed rule reflects a lack of understanding by FAA of how airlines operate," said Jim May, president of the trade group.

Prompted by growing concern from investigators at the National Transportation Safety Board (NTSB) and Congress over the role fatigue has played in recent crashes, the FAA in September unveiled a proposal that would give pilots longer rest periods and shorter workdays.

It for the first time called for more restrictions on pilots who work late at night or who make numerous flights, both of which cause more fatigue than flying a small number of flights during the day, according to scientists working for the FAA.

However, pilots who do not fly in those tiring conditions could fly longer hours, from the current eight-hour limit to as many as 10 hours.

The NTSB said it supports the proposal, though it urged FAA to closely monitor extending flight hours.

That increase in the allowable flight time prompted objections by some unions and dozens of individuals.

"There is no scientific data to support that increasing time on task will reduce fatigue," wrote the US Airline Pilots Association, which represents US Airways' pilots.

The Air Line Pilots Association, the largest pilot union, was more supportive of the proposal. It said the maximum time a pilot should fly per day was nine hours, but it mostly urged only minor changes.

The union also said airlines would exaggerate the expected costs of the proposal and urged the FAA to ignore the "sky is falling approach."

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